In the deep sub-micron generation in which design rules for a semiconductor device are ranged 0.3 to 0.2 .mu.m, thicknesses of the gate oxide film of a MOS transistor and a tunnel insulating film of a flash memory are respectively reduced down to as small as 10 nm or less and of the order of tens of nanometers, and a conductive material film such as a polysilicon film or a metal film is deposited on such very thin insulating films to cover, thus forming an electrode. Further, in a case where a so-called epitaxial wafer is produced by growing a silicon single crystal thin film on a surface of a silicon-based semiconductor substrate, a natural oxide film and impurities present on the surface of a silicon-based semiconductor substrate have to be all removed.
In the processes, a cleanliness of and the presence of a natural oxide film on the silicon-based semiconductor substrate directly play a major role on the device performance. Therefore, when a semiconductor device is formed on a silicon-based semiconductor substrate, a surface thereof is necessary to be cleaned to a high degree while suppressing a damage on the surface of the substrate to the lowest degree possible. Unintentional attachment of impurities and growth of an oxide film, however, occur on a surface of a silicon-based semiconductor substrate in a polishing step, a cleaning step and the like of substrate manufacture and in the course of storage in the air. That is, there occur attachment of organic materials and metal impurities, or growth of a natural oxide film on the surface of a silicon-based semiconductor substrate. Below, description will be made of conventional general removal processes for such impurities and natural oxide film. Organic materials attaching to the surface of a silicon-based semiconductor substrate have origins thereof in silanol, a vinyl chloride-based plasticizer in a clean room and the like and in order to remove the organic materials, there have been available two ways in a broad sense: wet cleaning and dry cleaning.
A typical example of the wet cleaning is the so-called SC-1 cleaning in which an alkaline cleaning solution (NH.sub.4 OH/H.sub.2 O.sub.2 /H.sub.2 O) is used.
The dry cleaning, on the other hand, is typically performed in a high temperature treatment in an atmosphere of a hydrogen (H.sub.2) gas or a mixed gas of H.sub.2 gas and a hydrogen chloride (HCl) gas. In addition to this, an ozone (O.sub.3) treatment, an ultraviolet irradiation treatment or a combination thereof have been known as a method that can be performed in the vicinity of room temperature. Among them, the high temperature treatment is advantageous in order to avoid regrowth of an oxide film. Among metal impurities attaching to a surface of silicon-based semiconductor substrate, metal impurities that are here regarded specifically problematic are heavy metals such as iron (Fe), nickel (Ni), chromium (Cr) and others. Such heavy metals are generally removed by the so-called SC-2 cleaning in which an acid cleaning liquid (HCl/H.sub.2 H.sub.2 /H.sub.2 O) is used, or alternatively the metals on and a surface region of the silicon-based semiconductor substrate are etched off together by holding the substrate in a H.sub.2 atmosphere at as high as of the order of 1100.degree. C. A process that is frequently employed in removal of a natural oxide film is to heat treat a substrate in an atmosphere constituted of H.sub.2 or a gas mixture of H.sub.2 /HCl at a temperature as high as in the vicinity of 1100.degree. C. In addition to this, as a process that can practically used in the vicinity of room temperature, there have been known wet etching using a dilute hydrofluoric acid solution, a combination of a dilute hydrofluoric acid treatment and a water vapor treatment and further, an Ar plasma treatment which is a dry treatment. However, various problems arise; an oxide film grows again immediately after removal of the natural oxide film, surface roughening of the substrate occurs, corrosion of a treatment facility and so on. Hence, the above described high temperature treatment is considered to be an optimal process at present. In a case where any of impurities is removed, however, when wet etching is adopted for the removal, pure water cleaning follows the wet etching in order to wash out a chemical liquid, further followed by drying and reattachment of impurities to a silicon-based semiconductor substrate cannot be avoided from occurring during the time the substrate is waiting for the next step after the drying. Further, there is also a risk that impurities attaching to a front surface of the silicon-based semiconductor substrate spread over to the rear surface through transportation by the chemical liquid and the cleaning water, which leads to a level at which impurity contamination widely affects the substrate.
In a process in which organic materials, metal impurities and a natural oxide film are removed by using H.sub.2 gas or a mixed gas of H.sub.2 /HCl at a high temperature in the vicinity of 1100.degree. C., a perfect removal of metal impurities, when the metal impurities are originally present on a surface of the substrate, has been hard to be achieved since the metal impurities are incorporated into the interior of the substrate by thermal diffusion. Especially, if a heavy metal is incorporated into the interior of silicon single crystal, the heavy metal works as causes for reduction of a carrier lifetime and formation of oxidation induced stacking faults. Therefore, the thermal diffusion as described above is required to be suppressed as much as possible.
In this way, it has conventionally been extremely difficult not only to suppress regrowth of a natural oxide film but also to prevent thermal diffusion of a metal impurity from occurring, both at the same time, and therefore, development of a process to simultaneously realize the suppression and prevention has been waited for. It is accordingly an object of the invention to provide such a cleaning process for a silicon-based semiconductor substrate.